Xcode is incredibly useful for debugging iOS apps, especially with the updates released in the last few years. Sometimes that isn’t enough. Sometimes you want to find the exact error that caused an exception throw, or only activate a breakpoint with a certain method previously in the call stack. These cases, which are hard to debug with Xcode’s standard toolset, are easy to debug with LLDB. This talk will walk through the foundations of debugging in LLDB for Objective-C and Swift, as well as introduce some more advanced features for those really tough bugs.

1. More Than Po:
Debugging In
lldb
@MicheleTitolo
Lead Software Engineer @ Capital One

2. lldb

3. What We’ll Cover
• LLDB basics
• Advanced thread, breakpoint,
watchpoint debugging
• Debugging with script

4. Lldb Commands

5. frame lang == lldb lang

6. Swift!

7. Basics

8. po

9. expr -O --

10. (lldb) po self.myCar
<Car: 0x7fc8206334d0>

11. (lldb) po self.canChangeGearTo(gear: Gear.Park)
true

12. -description

13. (lldb) po self.myCar
2014 Tesla S UIDeviceRGBColorSpace 0 0 1 1

14. p

15. expr --

16. (lldb) p self.myCar.year
(NSInteger) $1 = 2014

17. (lldb) p *self.myCar
(Car) $1 = {
NSObject = {
isa = Car
}
_running = NO
_make = 0x000000010077d088 @"Tesla"
_model = 0x000000010077d0a8 @"S"
_year = 2014
_color = 0x00007fd531eb9e30
_gear = Park
}

18. (lldb) po self.myCar
<SwiftCars.Car: 0x7fe8bb456a60>
(lldb) p *self.myCar
error: <EXPR>:1:1: error: '*' is not a prefix unary operator
*self.myCar
^

19. use expr to modify
values at runtime

20. (lldb) po self.myCar
2014 Tesla S UIDeviceRGBColorSpace 0 0 1 1
(lldb) expr self.myCar.year = 2013
(NSInteger) $0 = 2013
(lldb) po self.myCar
2013 Tesla S UIDeviceRGBColorSpace 0 0 1 1

21. frame variable

22. fr v

23. (lldb) fr v
(ViewController *) self = 0x00007f9628520cd0
(SEL) _cmd = "updateTipLabelsForBillAmount:"
(float) billAmount = 33
(float) tipPercentage = 0.200000003
(float) tipAmount = 6.5999999
(float) total = 39.5999985

24. (lldb) fr v -F self.myCar
self.myCar = 0x00007fe8bb456a60
self.myCar =
self.myCar.isa = SwiftCars.Car
self.myCar.make._core._owner = nil
self.myCar.model._core._owner = nil
self.myCar.color = 0x00007fe8bb452c80
self.myCar.color.isa = UICachedDeviceRGBColor
self.myCar.color._systemColorName = 0x00007fe8bb4424c0
“blueColor"
self.myCar.color.redComponent = 0
self.myCar.color.greenComponent = 0
self.myCar.color.blueComponent = 1
self.myCar.color.alphaComponent = 1
self.myCar.color.cachedColor = 0x0000000000000000
self.myCar.color.cachedColorOnceToken = 0

25. bt

26. thread backtrace

27. (lldb) bt
* thread #1: tid = 0x66c2, 0x0000000101a90330 Cars`-
[ViewController viewDidLoad](self=0x00007fc3d2c21570,
_cmd=0x0000000102f21cfb) + 448 at ViewController.m:28, queue =
'com.apple.main-thread', stop reason = breakpoint 1.1
* frame #0: 0x0000000101a90330 Cars`-[ViewController
viewDidLoad](self=0x00007fc3d2c21570, _cmd=0x0000000102f21cfb) +
448 at ViewController.m:28
frame #1: 0x000000010284f090 UIKit`-[UIViewController
loadViewIfRequired] + 738
frame #2: 0x000000010284f28e UIKit`-[UIViewController view] +
27
frame #3: 0x000000010276b5e9 UIKit`-[UIWindow
addRootViewControllerViewIfPossible] + 58
frame #4: 0x000000010276b9af UIKit`-[UIWindow
_setHidden:forced:] + 247

28. step / s

31. next / n

34. thread step-out /
finish

37. continue / c

39. ~/.lldbinit

40. settings set prompt [lldb]$
command alias bd breakpoint disable
command alias be breakpoint enable
command alias bdel breakpoint delete
command alias bcommand breakpoint command add
command alias commands breakpoint command list

41. process

42. (lldb) process attach -p 5416
Process 5416 stopped
* thread #1: tid = 0xe4b02, 0x0000000106e234de
libsystem_kernel.dylib`mach_msg_trap + 10, queue = 'com.apple.main
thread', stop reason = signal SIGSTOP
frame #0: 0x0000000106e234de libsystem_kernel.dylib`mach_msg_trap +
10
libsystem_kernel.dylib`mach_msg_trap:
-> 0x106e234de <+10>: retq
0x106e234df <+11>: nop
libsystem_kernel.dylib`mach_msg_overwrite_trap:
0x106e234e0 <+0>: movq %rcx, %r10
0x106e234e3 <+3>: movl $0x1000020, %eax
Executable module set to "/Users/michele/Library/Developer/CoreSimulator/Devices/
5304657F-1710-42A1-87CA-E5358E87F4C8/data/Containers/Bundle/Application/312BA401-3B1E-4A17-
B7E8-BAAAE13EC463/Cars.app/Cars".
Architecture set to: x86_64h-apple-ios.
(lldb) c
Process 5416 resuming

44. help

45. help breakpoint

46. help breakpoint
command

47. help breakpoint
command add

48. -h --help

49. Thread

50. thread info

51. (lldb) thread info
thread #1: tid = 0x66c2, 0x0000000101a90330 Cars`-[ViewController
viewDidLoad](self=0x00007fc3d2c21570, _cmd=0x0000000102f21cfb) +
448 at ViewController.m:28, queue = 'com.apple.main-thread', stop
reason = breakpoint 1.1

52. thread list

53. (lldb) thread list
Process 921 stopped
* thread #1: tid = 0x66c2, 0x0000000101a90330 Cars`-
[ViewController viewDidLoad](self=0x00007fc3d2c21570,
_cmd=0x0000000102f21cfb) + 448 at ViewController.m:28, queue =
'com.apple.main-thread', stop reason = breakpoint 1.1
thread #2: tid = 0x66da, 0x0000000104bcc232
libsystem_kernel.dylib`kevent64 + 10, queue =
'com.apple.libdispatch-manager'
thread #3: tid = 0x66dc, 0x0000000104bc651a
libsystem_kernel.dylib`semaphore_wait_trap + 10, queue =
'FBSSerialQueue'
thread #4: tid = 0x66dd, 0x0000000104bcb94a
libsystem_kernel.dylib`__workq_kernreturn + 10
thread #5: tid = 0x66de, 0x0000000104bcb94a
libsystem_kernel.dylib`__workq_kernreturn + 10

54. thread until
<linenum>

55. (lldb) thread until 31
Process 1615 resuming
(lldb)

56. thread return
<expr>

57. (lldb) thread return NO
(lldb) thread return [NSNumber numberWithInt:7]
(lldb) thread return @"Hello"

58. (lldb) thread return true
error: Error returning from frame 0 of thread 1: We only support
setting simple integer and float return types at present..

59. Breakpoints

61. breakpoint list

62. br list

63. (lldb) br list
Current breakpoints:
1: file = 'Cars/Car.m', line = 50, locations = 1, resolved = 1,
hit count = 0
1.1: where = Cars`-[Car changeGearTo:] + 52 at Car.m:50,
address = 0x00000001026fd4e4, resolved, hit count = 0
2: file = 'Cars/ViewController.m', line = 31, locations = 1,
resolved = 1, hit count = 1
2.1: where = Cars`-[ViewController viewDidLoad] + 512 at
ViewController.m:31, address = 0x00000001026fcb40, resolved, hit
count = 1

64. br set
br modify
br delete

65. (lldb) br set -f ViewController.m -l 31
Breakpoint 2: where = Cars`-[ViewController viewDidLoad] +
512 at ViewController.m:31, address = 0x000000010e2bdb40

66. (lldb) br set -F "-[Car changeGearTo:]”
Breakpoint 3: where = Cars`-[Car changeGearTo:] + 20 at
Car.m:47, address = 0x00000001073ba4c4

67. (lldb) br set -F “SwiftCars.Car.changeGearTo(gear: Cars.Gear) -> ()”
Breakpoint 10: where = SwiftCars`SwiftCars.Car.changeGearTo(gear:
SwiftCars.Gear) -> () + 18 at ViewController.swift:35, address =
0x000000010c97e032

68. SwiftCars.Car.changeGearTo(gear: SwiftCars.Gear) -> ()
Module Name
Type
Function signature
Namespaced

69. SwiftCars.Car.increaseSpeedTo(speed: Swift.Int) -> ()
Built-in types are in the Swift module

70. br ~= b

71. (lldb) b ViewController.m:31

72. (lldb) br ViewController.m:31
error: command 'breakpoint' did not recognize 'ViewController .m:
31' as valid (subcommand might be invalid).

73. (lldb) b Car.m:63
Breakpoint 2: where = Cars`-[Car increaseSpeedTo:] + 118 at
Car.m:63, address = 0x0000000100b7a576
(lldb) br modify -c "speed==15" 2
(lldb) c
Process 3093 resuming
(lldb)

75. (lldb) br modify -c "speed==15 && self.gear==4” 2

76. (lldb) br modify -c "speed==15 && gear==Gear.Drive" 1

77. (lldb) br modify -c "speed==20 && [self canChangeGearTo:4]" 2

78. (lldb) br modify -c "speed==15 && self.gear==4" -i 17 2

79. -i <count> ( --ignore-count <count> )
Set the number of times this
breakpoint is skipped before stopping.

80. -i <count> ( --ignore-count <count> )
Set the minimum number of times this
breakpoint is skipped before stopping.

81. (lldb) br delete 2
1 breakpoints deleted; 0 breakpoint locations disabled.

82. br enable
br disable

83. (lldb) breakpoint disable 2
1 breakpoints disabled.
...
2: file = 'Cars/ViewController.m', line = 31, locations = 1
Options: disabled

84. (lldb) breakpoint enable 2
1 breakpoints enabled.
...
2: file = 'Cars/ViewController.m', line = 31, locations = 1,
resolved = 1, hit count = 1

85. br command

86. (lldb) br command add -o "fr v" 2
(lldb) fr v
(Car *) self = 0x00007fb59ad142b0
(SEL) _cmd = "changeGearTo:"
(Gear) gear = Neutral

87. (lldb) br command add 1
Enter your debugger command(s). Type 'DONE' to end.
> fr v
> continue
> DONE
(lldb) fr v
(Car *) self = 0x00007f8061625490
(SEL) _cmd = "changeGearTo:"
(Gear) gear = Reverse
(lldb) continue
Process 2068 resuming
Command #2 'continue' continued the target.

88. (lldb) br command list 3
Breakpoint 3:
Breakpoint commands:
fr v

89. (lldb) br command delete 3

90. Breakpoints Added In
Console Do Not Persist
Between Runs

91. Watchpoints

93. A Watchpoint Tracks
A Value Over Time

94. …Kind Of Like Kvo!

95. [self.myCar addObserver:self
forKeyPath:@"speed"
options:NSKeyValueObservingOptionNew
context:KVOContext];
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object
change:(NSDictionary *)change context:(void *)context
{
if (car.speed > 1000) {
NSLog(@"WHY AM I GOING SO FAST %ld",
(long)car.speed);
}
}

96. - (void)setSpeed:(NSInteger)speed
{
_speed = speed;
if (_speed > 1000) {
NSLog(@"WHY AM I GOING SO FAST %ld",
(long)_speed);
}
}

97. breakpoint: place
watchpoint: value

98. watchpoint set

99. watch set

100. (lldb) watch set var self.speed
error: "self" is a pointer and . was used to attempt to
access "speed". Did you mean "self->speed"?
(lldb) watch set var self->speed
error: "speed" is not a member of "(Car *const) self"
(lldb) watch set var _speed

101. (lldb) watch set var self.speed
Watchpoint created: Watchpoint 1: addr = 0x6080000a4698 size = 8
state = enabled type = w
declare @ ‘/Projects/SwiftCars/ViewController.swift:28’
watchpoint spec = 'self.speed'

102. watch modify -c …
-i …

103. (lldb) watch set var _speed
Watchpoint created: Watchpoint 1: addr = 0x7fb9c145d768 size
= 8 state = enabled type = w
watchpoint spec = ‘_speed'
new value: 0
(lldb) watch modify -c '(_speed==15)'
(lldb) c
Process 2150 resuming
Watchpoint 1 hit:
old value: 0
new value: 15
(lldb)

104. (lldb) watch set var self.speed
Watchpoint created: Watchpoint 1: addr = 0x6080000a4698 size = 8
state = enabled type = w
declare @ ‘/Projects/SwiftCars/ViewController.swift:28’
watchpoint spec = 'self.speed'
(lldb) watch modify -c ‘(self.speed==15)’ 1
(lldb) c
Watchpoint 1 hit:
(lldb) po self.speed
6
❌

105. (lldb) watch command add 1
Enter your debugger command(s). Type 'DONE' to end.
> p _speed
> continue
> DONE
(lldb) c
Process 2252 resuming
(NSInteger) $16 = 15
Process 2252 resuming
Command #2 'continue' continued the target.

106. watch delete
watch enable
watch disable

107. (lldb) watch disable 1
1 watchpoints disabled.
(lldb) watch enable 1
1 watchpoints enabled.

108. Script

110. write code to debug
your code

111. copy + paste

113. script

114. (lldb) help script
Pass an expression to the script interpreter for evaluation and
return the results. Drop into the interactive interpreter if no
expression is given. This command takes 'raw' input (no need to
quote stuff).
Syntax: script [<script-expression-for-evaluation>]

115. (lldb) script
Python Interactive Interpreter. To exit, type 'quit()', 'exit()'.
>>> print lldb.debugger
Debugger (instance: "debugger_47", id: 47)
>>> print lldb.target
Cars
>>> print lldb.process
SBProcess: pid = 6425, state = stopped, threads = 5, executable =
Cars
>>> print lldb.thread
SBThread: tid = 0x3813e
>>> print lldb.frame
frame #0: 0x0000000103075979 Cars`-[ViewController viewDidLoad]
(self=0x00007f8173f23680, _cmd=0x0000000104507cfb) + 57 at
ViewController.m:24
>>> quit()

116. Writing functions

117. def breakpoint_func(frame, bp_loc, dict):
# implementation here
# return false to skip
current stack frame breakpoint locationpython session dictuseless

118. Sample Case

119. only stop when
-changeGearTo:
is in the call stack

120. find_in_stack = ['-[Car changeGearTo:]']
def continue_ignored(frame, bp_loc, dict):
global find_in_stack
names = set([frame.GetFunctionName() for frame
in frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
quit()
find_in_stack = ['-[Car changeGearTo:]']
def continue_ignored(frame, bp_loc, dict):
global find_in_stack
names = set([frame.GetFunctionName() for frame
in frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
quit()

121. find_in_stack = ['-[Car changeGearTo:]']
def continue_ignored(frame, bp_loc, dict):
global find_in_stack
names = set([frame.GetFunctionName() for frame
in frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
quit()
find_in_stack = ['-[Car changeGearTo:]']
def continue_ignored(frame, bp_loc, dict):
global find_in_stack
names = set([frame.GetFunctionName() for frame
in frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
quit()
stack symbol
function declaration
global accessor
all functions in stack
check if symbol we want
is in this stack
if it isn’t, continue

122. find_in_stack = ['-[Car changeGearTo:]']
def continue_ignored(frame, bp_loc, dict):
global find_in_stack
names = set([frame.GetFunctionName() for frame
in frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
quit()

123. find_in_stack = ['SwiftCars.Car.changeGearTo']
Module

124. br command add -s
python #

125. (lldb) br command add -s python 2
Enter your Python command(s). Type 'DONE' to end.
def function(frame,bp_loc,internal_dict):
"""frame: the SBFrame for the location at which you stopped
bp_loc: an SBBreakpointLocation for the breakpoint
location information
internal_dict: an LLDB support object not to be used"""
global find_in_stack
find_in_stack = ['-[Car changeGearTo:]']
names = set([frame.GetFunctionName() for frame in
frame.GetThread()])
all_ignored = set(find_in_stack)
ignored_here = all_ignored.intersection(names)
if len(ignored_here) == 0:
return False
DONE

127. (lldb) br command add -s python 2 -o “print 1+1”

128. type summary

129. Use A Python
Function To Better
Describe A Class

130. Swift

131. def Car_Summary(value,unused):
make = value.GetChildMemberWithName("_make")
model = value.GetChildMemberWithName("_model")
makeSummary = make.GetSummary()
modelSummary = model.GetSummary()
return makeSummary + " " + modelSummary
quit()

132. def Car_Summary(value,unused):
make = value.GetChildMemberWithName("_make")
model = value.GetChildMemberWithName("_model")
makeSummary = make.GetSummary()
modelSummary = model.GetSummary()
return makeSummary + " " + modelSummary
quit()
def Car_Summary(value,unused):
make = value.GetChildMemberWithName("_make")
model = value.GetChildMemberWithName("_model")
makeSummary = make.GetSummary()
modelSummary = model.GetSummary()
return makeSummary + " " + modelSummary
quit()
value is the frame
get the variables we want
printable
summaries
return a string

133. def Car_Summary(value,unused):
make = value.GetChildMemberWithName("_make")
model = value.GetChildMemberWithName("_model")
makeSummary = make.GetSummary()
modelSummary = model.GetSummary()
return makeSummary + " " + modelSummary
quit()

135. More Use Cases
• Only break after another breakpoint has
been hit
• Check multiple threads for a symbol
• Data formatters for everything in Objc
• Custom LLDB commands

136. Questions?
@MicheleTitolo

137. • WWDC ’13 Session 413
• WWDC ’14 Sessions 409 & 410
• http://lldb.llvm.org/
• http://www.objc.io/issue-19/lldb-debugging.html
• http://blog.ittybittyapps.com/blog/2013/11/07/
integrating-reveal-without-modifying-your-xcode-
project/
• https://github.com/facebook/chisel
• https://developer.apple.com/library/mac/
documentation/IDEs/Conceptual/
gdb_to_lldb_transition_guide/document/lldb-terminal-
workflow-tutorial.html
Resources

138. • https://unsplash.com/photos/
eMnddgd3pjQ
• https://unsplash.com/photos/
KpBAYMNf9Tw
Photo Credits